1. Field of the Disclosure
The invention pertains to a method and apparatus for mounting a printing form onto a cylindrically shaped base, and in particular the method and apparatus mounts a plurality of cylindrically shaped printing forms onto the base.
2. Description of Related Art
In relief printing processes a flexible printing form, usually in the shape of a plate, is mounted on a print cylinder of a printing press, inked, and then contacted to a substrate to print the desired pattern on the substrate.
Flexible printing plates are conventionally mounted on the print cylinder of a printing press by one of the following two methods in flexographic printing: (1) a carrier sheet, having one or more printing plates attached, is wrapped around the print cylinder; (2) one or more printing plates are directly mounted on the print cylinder, or onto a print sleeve that is mounted on the print cylinder. The plate or plates are attached securely to the carrier sheet or print cylinder using double-sided tape. However, in each method the plate or plates must be mounted accurately on the carrier sheet or print cylinder in order to assure registration of the printed image on the substrate.
The accurate positioning or registering of the flexible printing form on the surface of a printing cylinder has always been an important element in the attainment of quality printing. In particular, the accurate positioning or registration of each printing form on each cylinder is of paramount importance in a situation where multiple printing forms are required for the production of a multiple color finished product and where each color is applied to the product in a sequential manner by different forms on different cylinders. Registration errors give rise to superimposed colors, spaces with no color, color shifts, and/or degraded image detail.
The quality of a flexographic printing job depends, in large measure on the care with which prepress preparations are carried out. Plate mounting, color registration, and proofing can be conducted off the press by means of commercially available mounting-proofing machines designed for this purpose. These machines, which usually make use of an optical or video mounting system, make it possible to mount the plates on the plate cylinder to effect color registration, a procedure essential to the maintenance of both quality and economy in all flexographic operations.
However, flexible printing forms in the shape of plates may not be suitable for some printing jobs. Flexible printing forms that are cylindrically shaped, which may be referred to as continuous printing forms, sleeves, printing sleeves, or endless sleeves, have use in particular applications and provide certain advantages. Continuous cylindrically shaped printing forms have applications in the flexographic printing of continuous designs such as in wallpaper, decoration and gift wrapping paper, and tight-fit conditions for registration, since the designs can be easily printed without print-through of the plate seam. An alternative embodiment of a cylindrically-shaped printing form is sometimes referred to as a plate-on-sleeve, where one or more plates, or portions of a plate (sometimes called slugs) are mounted at various spaced locations on a cylindrically shaped support. The ends of a plate or a portion of a plate may or may not meet or join when wrapped onto the support. Such continuous printing forms and plate-on-sleeve forms are well-suited for mounting on laser exposure equipment where it can replace the drum or be mounted on the drum for exposure by a laser to achieve precise registration.
Continuous printing sleeves or cylindrically-shaped supports are generally mounted on a print cylinder by means of pressurized air that is injected at one end of the print cylinder and emerges through opening/s at an exterior surface of the print cylinder usually arranged angularly around the cylinder. This creates a cushion of air between the print cylinder and the continuous printing sleeve as the sleeve is mounted on the cylinder, which causes a slight radial expansion of the sleeve and allows the sleeve to move along and around the print cylinder. A single continuous printing form may be mounted in register on the print cylinder by a mounter-proofer unit as is done for plates. Once the sleeve is in position, the air is turned off so that the sleeve contracts and forms an interference fit with the print cylinder.
However, a problem arises in the case where more than one cylindrically shaped printing form will be mounted in register on the same print cylinder. A first cylindrically shaped printing form can easily be positioned in its registration position with the air cushion, and then retain the registration position when the printing form contracts with air turned off to the exterior surface of the print cylinder. However, the mounting of any additional cylindrically shaped printing form on the same print cylinder needs the air cushion to move the additional forms to registration position/s on the print cylinder. But the introduction of the air cushion can move the first printing form from its registration position. Thus, it is difficult to maintain registration of the first mounted (or any previously mounted) printing form while subsequently mounting any additional printing form/s on the same print cylinder.
Mechanical means have been devised to accommodate registration mounting of more than one cylindrically shaped printing form as disclosed in EP 0 510 744. EP 0 510 744 discloses an apparatus for mounting one or plurality of printing sleeves onto a printing roll core. The printing sleeves are mounted on the core from one end and are moved to a number of register positions on the core. The register positions include a number of registration means, such as pins, that project from an exterior surface of the core and cooperate with at least one recess in an edge of the sleeves. But the use of such mechanical pin means can be cumbersome and require modification of the printing cylinder to accommodate the mechanical registration pins.
Schadlich et al. in U.S. Pat. No. 5,551,339 disclose a process and device for register-correct positioning of a printing form sleeve on a printing cylinder at each print station of a rotary printing machine, each printing cylinder having a pressure gas cushion producible for moving the elastically-expandable printing form sleeve on the printing cylinder. At each print station, a printing form sleeve is placed on its respective printing cylinder without particular regard to its correct registration position. The position of the tightly (frictionally) fitted sleeve on the printing cylinder is determined by scanning the sleeve with a sensor to determine the registration markings relative to a register-correct position on the printing cylinder. The printing sleeve is released from its initially fitted position on the printing cylinder using the pressure gas cushion and held by a holding device. Then the printing cylinder is turned relative to the printing form sleeve (being held in position) by its angular deviation relative to its register-correct position. Subsequently the pressure gas cushion is turned off, and the printing form sleeve is again tightly fitted and set in its register-correct position relative to the printing cylinder. This method registers multicolor image produced by multiple printing stations each having a single printing form sleeve on a respective printing cylinder. However, the method does not acknowledge the problem associated with mounting in registration multiple printing form sleeves on the same printing cylinder.
Thus, it is important that each of more than one flexible cylindrically shaped print forms be mounted in register on a print cylinder. But the necessary presence of the air cushion to mount each additional cylindrical printing form on a print cylinder can change the position of the initially mounted cylindrical printing form out of registration on the cylinder. As such, it is difficult to maintain registration of the first mounted (or any previously mounted) printing form while subsequently mounting with an air cushion any additional printing form/s on the same print cylinder.